1. Field of the Invention
The present invention relates generally to protective coatings, and more particularly to a corrosion, oxidation, and high temperature resistant layered coating for gas turbine engine materials.
2. Description of the Prior Art
Gas turbine engines operate under extreme conditions of temperature, pressure, and environment, and therefore have a high rate of degradation. Peak operating temperatures typically reach between 1,000 and 2,000 degrees Fahrenheit. These high temperatures cause the surfaces of turbine engine materials to oxidize and degrade. Thermal cycling, the rapid temperature changes during start up and shut down, causes further oxidation, cracking and flaking of the surface, and degradation of engine materials. In addition, gas turbine engines often operate in corrosive environments, such as ocean based naval operations. At the high temperatures and pressures experienced in gas turbine engines, salts from these environments become molten and highly corrosive. The gas turbine blades and vanes are particularly susceptible to high temperature, corrosion, and degradation from these molten salts. Gas turbine engines must therefore be constantly monitored and maintained, replacing expensive parts which have degraded.
Various protective ccatings are known for reducing degradation in high temperatures and corrosive environments. Aluminide coatings, (a nickel, cobalt, aluminum intermetallic compound) MCrAlY coatings (where M is a metal such as nickel or cobalt alone or in combination), and ceramic thermal barrier coatings, alone or in combination, are currently used to protect gas turbine blades and vanes from exposure to these operating conditions.
Nickel oxide coatings have also been previously studied and found to provide excellent protection from molten salts in a high temperature environment. However, if contaminated with other elements such as aluminum or chromium, a nickel oxide coating will rapidly degrade in a high temperature, corrosive environment. The combination of a nickel oxide coating with a substrate or metallic alloy containing these elements has therefore previously not been available, as some contamination is an unavoidable consequence of applying or forming a nickel oxide to such substrates or metal alloys. In addition, the thermal expansion of the substrate materials currently used in gas turbine blades and vanes is substantially greater than the thermal expansion of nickel oxide. A nickel oxide coating will therefore crack and flake off when applied to such metallic alloy substrates.
For these and other reasons. presently known high temperature and corrosion resistant coatings for metallic alloys have not included a nickel oxide and MCrAlY combination. A need exists for a protective coating for turbine engine parts having improved high temperature and corrosion resistance.